I believe it just has to do with how many atoms of Uranium will decay at a time. More or less, the uranium from the fuel rods are throwing out a bunch of neutrons that collide with other atoms, and those atoms split apart and release energy and other neutrons, which go on to collide with other atoms in a chain reaction. As time goes on, more and more of these atoms decay into daughter elements that are more stable, so the fuel very slowly starts to become less efficient. I don’t know at what point they decide they need to refuel, but the reason why we don’t use all of it is because you would have to wait for a very long time while using a less and less powerful fuel source. Nuclear is getting more and more efficient though, and other fuel is being used in some reactors. I’ve heard about the recycling thing, but I’m not sure how they do it